Piezoresistive pressure transducer circuitry accommodating transducer variability

ABSTRACT

Piezoresistive pressure transducer circuitry accommodating transducer variables comprising microminiature pressure transducer having first and second variable resistive elements each having first and second ends, circuitry coupled to the first and second resistive elements of the transducer including third and fourth fixed resistive elements each having a first and second ends, means interconnecting the first ends of the first and second resistive elements, means interconnecting the second ends of the first and third resistive elements, means interconnecting the second ends of the second and fourth resistive elements, means interconnecting the first ends of the third and fourth resistive elements, means for supplying excitation to the first ends of the third and fourth resistive elements, first amplifier means having an input coupled to the means interconnecting the second ends of the first and third resistive elements, second amplifier means having an input coupled to the means interconnecting the second ends of the second and fourth resistive elements. First analog-to-digital converter having an input coupled to the output of the first amplifier means, a second analog-to-digital converter having an input coupled to the output of the second amplifier means, a computer and means coupling the computer to the outputs of the first and second analog-to-digital converters.

This invention relates to a piezoresistive transducer circuitryaccommodating transducer variability and having reduced dynamic rangerequirements.

Conventional pressure transducer systems, both strain gauge andpiezoresistive types, typically utilize a balanced Wheatstonebridge-type circuit between two variable and two fixed resistiveelements. The transducer itself may contain only the two variableresistive elements or all four resistive elements. Typically theresistive elements are trimmed during manufacture to ensure a closetolerance to and match between the nominal zero-pressure resistancevalues. The resistive elements are also trimmed to provide a close matchbetween individual pressure-resistance characteristics. Electricalcircuitry typically utilized therewith permits a small adjustment to thevalue of one of the fixed resistors by adding a small offset voltage toone of the output legs of the bridge. The demodulated amplified outputcan then be supplied to an analog-to-digital converter. With suchcircuitry, typically the dynamic range of the amplified signal ismatched to the input of the analog-to-digital converter and only 9 bits(1 part in 512) of resolution is needed in the analog-to-digitalconverter to provide adequate pressure resolution (resolution betterthan 1 mmHg from 0-300 mmHg or over a 0-300 mmHg span as typicallyrequired for human blood pressure measurements). With the advent ofultraminiature pressure sensors such as that disclosed in the co-pendingapplication Ser. No. 08/300,445 filed on Sep. 2, 1994, it has been foundthat the conventional Wheatstone circuitry of the type hereinbeforedescribed is incapable of assimilating the various characteristics ofsuch ultraminiature pressure sensors such as insensitivity to pressure,variability of resistor values, sensitivity to temperature, and withpressure sensitivity decreasing inversely with temperature increases.There is therefore need for a new and improved circuitry for use withsuch ultraminiature pressure sensors which can accommodate suchvariabilities.

In general, it is an object of the present invention to providepiezoresistive pressure transducer circuitry and method accommodatingtransducer variability.

Another object of the invention is to provide circuitry which makes itpossible to measure the pressure component and the temperature componentof the change in resistance of each variable resistor.

Another object of the invention is to provide a circuitry and method ofthe above character which makes it possible to make independentmeasurements of the variable resistors as well as differentialmeasurements.

Another object of the invention is to provide circuitry and method ofthe above character which has reduced dynamic range requirements.

Another object of the invention is to provide circuitry and method ofthe above character which makes use of easily obtainable inexpensivecomponents.

Another object of the invention is to provide circuitry and method ofthe above character in which a computer can be utilized to compensatefor variability in the resistors.

Another object of the invention is to provide circuitry and method ofthe above character which can compensate in real time for undesiredcharacteristics of the transducer.

Another object of the invention is to provide circuitry and method ofthe above character in which the dynamic range requirements have beenreduced by computer generated feedback in real time.

Additional objects and features of the invention will appear from thefollowing description in which the embodiments are set forth in detailin conjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram of a modified bridge circuitincorporating the present invention in use with an ultraminiaturepressure transducer.

FIG. 2 is a schematic block diagram of another modified bridge circuitincorporating the present invention for use with an ultraminiaturepressure transducer which makes possible a dramatic reduction inanalog-to-digital converter dynamic range requirements.

FIG. 3 is a schematic block diagram of still another modified bridgecircuit showing an alternative circuit and method to accomplish theresults achieved in the circuit shown in FIG. 2.

In general the circuitry incorporating the present invention is for usewith an ultraminiature piezoresistive pressure transducer having firstand second resistive elements, each having first and second ends. Thecircuitry comprises third and fourth resistive elements each havingfirst and second ends. Means is provided for connecting the first endsof the first and second resistive elements. Means is provided forconnecting the second ends of the first and third resistive elements.Means is also provided for connecting the second ends of the second andfourth resistive elements. Means is provided for connecting the firstends of the third and fourth resistive elements. Excitation means isprovided which is connected to the first ends of the third and fourthresistive elements. First amplifier means is provided having an inputcoupled to the means for connecting the second ends of the first andthird resistive elements. Second amplifier means is provided having aninput coupled to the means for connecting the seconds ends of the secondand fourth resistive elements. First and second analog-to-digitalconverters are connected respectively to the outputs of the first andsecond amplifier means. Computer means is connected to the first andsecond analog-to-digital converters for making adjustments in real timeto take into account the variabilities of the first and second resistiveelements and to perform calculations to result in an output whichrepresents the pressure being measured by the ultraminiaturepiezoresistive pressure transducer.

More in particular as shown in FIG. 1 of the drawings, the circuitry 11incorporating the present invention is for use with a ultraminiaturepiezoresistive pressure transducer 12. The pressure transducer 12 is ofthe type described in copending application Ser. No. 08/300,445 filed onSep. 2, 1994. As described therein, the transducer 12 is sized so thatit can be mounted on the distal extremity of a flexible elongate elementsuch as a guide wire having a diameter of 0.018" and less. The guidewire is for use in diagnostic and therapeutic procedures using cathetersand in which the catheters are utilized with such a guide wire, as forexample in an angioplasty procedure. The piezoresistive transducer ismounted in a pressure sensor assembly which incorporates thepiezoresistive transducer in a diaphragm structure formed of a suitablecrystalline material such as silicon and is fabricated from a die havinga length of 1050 microns and width of 250 microns for a 0.014" guidewire, and a width between 250 and 350 microns for a 0.018" guide wireand thickness of 50 microns. First and second resistive elements areformed in the die as described in said pending application Ser. No.08/300,445 filed on Sep. 2, 1994. These two variable resistorsidentified as variable resistors R_(A) and R_(B) are provided with firstand second ends identified by the numerals of 1 and 2. The two resistiveelements R_(A) and R_(B) are configured so they extend over a flexiblesilicon diaphragm which upon the application of pressure to diaphragmcauses one resistive element to increase in resistive value while theother resistive element decreases in value. Typical characteristics forsuch a piezoresistive pressure transducer which are involved in thedesign of the circuitry of the present invention include:

    ______________________________________                                        Nominal resistance, each                                                                       3500Ω ± 1000Ω                                 resistor                                                                      Pressure sensitivity (K.sub.P)                                                                 1.25Ω to 15Ω/1000Ω/100 mmHg                Temperature coefficient                                                                        +28% ± 7.5%/100° C.                                of resistance (K.sub.T)                                                       Temperature coefficient                                                                        -20% ± 7.5%/100° C.                                of pressure sensitivity (K.sub.S)                                             Matching between resistors                                                                     None                                                         Optimal resistor power                                                                         0.035 to 0.07 mW                                             dissipation (each resistor)                                                   ______________________________________                                    

From the above specifications, it can be seen that the sensitivity ofsuch an ultraminiature piezoresistive transducer to pressure can beequal to, in the best case, or in the worst case, approximately 20 timesless than the temperature sensitivity, an unwanted characteristic.

In accordance with the present invention the voltage across eachtransducer resistor R_(A) and R_(B) must be measured independently. Bothmeasurements are needed to simultaneously solve two equations, eachhaving two unknowns P (pressure) and T (temperature). Other parametersof the transducer, including the nominal resistance R₁, and thesensitivity factors K_(P), K_(T), and K_(S), must be known and aretypically measured during the manufacturing calibration process.

The circuitry for accomplishing this is shown in FIG. 1 and includes twoexternal fixed resistors R_(F1) and R_(F2), each having first and secondends identified as 1 and 2. As shown in FIG. 1, means in the form of aconductor 16 is provided for connecting the first ends of the first andsecond variable resistive elements R_(A) and R_(B) which are connectedthrough a return path to a ground 17. Means in the form of a conductor19 is provided for connecting the second end of the first variableresistor R_(A) and the second end of the first fixed resistor R_(F1) toform a first junction. Means in the form of a conductor 21 is providedfor connecting the second end of the variable resistor R_(B) to thesecond fixed resistor R_(F2) to form a second junction. Means in theform of a conductor 23 is provided for connecting the first end of thefirst fixed resistor R_(F1) to the first end of the second fixedresistor R_(F2) and are connected to an excitation voltage source 26. Afirst independent amplifier means 31 is provided which has its inputconnected by a conductor 32 to the conductor 19 and is provided with anoutput 33. A second separate and independent amplifier means 36 has aninput connected by a conductor 37 to the conductor 21 and has an output38. First and second analog-to-digital converters 41 and 42 are providedwhich are connected respectively to the outputs 33 and 38 of theamplifiers 31 and 36. A computer 46 is provided which is connected tothe analog-to-digital converters 41 and 42 through a communication link47 as shown in FIG. 1 for reading the digitized voltages from theanalog-to-digital converters 41 and 42 and computing therefrom thepressure being measured by the transducer 12. The computer can be of asuitable type such as a 16-bit microprocessor.

From the foregoing description it can be seen that two independentamplifiers 31 and 36 are utilized to feed the signals to two independentanalog-to-digital converters which are connected to the computer 46.This makes it possible to provide the information which is needed tosolve simultaneously two equations with two unknowns. Each voltagemeasured has a pressure component and a temperature component. Bymeasuring these two components individually, it is possible to performthe necessary calculation for the simultaneous equations to resolve bothparameters as set forth below.

As soon as each voltage has been measured, the variable resistancesR_(A) and R_(B) can be calculated when the excitation voltage and thevalues of the fixed resistors R_(F1) and R_(F2) are known. Thetemperature-compensated pressure value may then be computed as follows.At any instant, the transducer resistance R (for each resistor) is afunction of six parameters: the initial resistance R₁ at the calibrationtemperature and pressure; the applied pressure P; the pressuresensitivity K_(P) ; the temperature T; the temperature coefficient ofthe resistance K_(T) ; and the temperature coefficient of the pressuresensitivity K_(S). The equation to express this function is:

    R=R.sub.1 +K.sub.T T+(K.sub.P P)(1-K.sub.S T)

Of the above variables, all are known except P and T, but since themeasurements of the two independent resistors (each with its own R₁,K_(T), K_(P), and K_(S)), have been ascertained, two equations as abovefor R_(A) and R_(B) can be solved for P and T. This is done by solvingfor P in terms of only one of the resistors and its parameters, andsolving for T in the terms of the other: ##EQU1##

Then the equation for T is substituted into the equation for P. Solvingthe resulting equation for P results in a quadratic equation, having thesolution: ##EQU2## into which, for brevity, the following terms havebeen substituted: ##EQU3##

From the foregoing it can be seen that the temperature compensatedpressure value for the ultraminiature piezoresistive transducer 12 canbe ascertained. However, in addition, to meet the requirements ofpractical and inexpensive circuitry operating with such ultraminiaturepiezoresistive transducer there is a great need to reduce the dynamicrange requirements (the span between the smallest and largest signals)of such ultraminiature piezoresistive transducer. By way of example,utilizing an ultraminiature piezoresistive transducer 12 of the typedescribed in co-pending application Ser. No. 08/300,445 filed on Sep. 2,1994 having the transducer characteristics as set forth above, thepressure signal from a low sensitivity transducer with a nominalexcitation voltage of one volt over the pressure range 0 to 300 mmHg,can be calculated as 836 μV, or 2.8 μV/mmHg. The maximum signalgenerated by the extremes of the ultraminiature piezoresistivetransducer characteristics over the required pressure and temperatureranges, will be approximately 190 mV (most of this coming from thevariance of the transducer resistance from the ideal 3500Ω). Thus, toresolve to 1 mmHg, it is necessary to resolve as a minimum, 1 part in68000 (190 mV/2.8 μV). If an analog-to-digital converter is utilized todigitize this signal, an accuracy of 17 bits is minimum. To provideconventional circuitry which could be utilized with such low sensitivitytransducers would be very expensive at best. Great amplification wouldbe required which would also amplify the offset voltages. It would bedifficult to provide amplifiers having sufficient dynamic range. Even ifthat were possible, it would be necessary to provide analog-to-digitalconverters also having a large dynamic range in order to resolve thepressure signal from the very large offset voltage. Circuitry toovercome the above identified difficulties providing for a reduction ofanalog-to-digital converter dynamic range requirements is set forth inFIG. 2.

The circuitry 51 shown in FIG. 2 incorporates a bridge arrangementsimilar to that shown in FIG. 1 with the variable resistors R_(A) andR_(B) in the transducer 12 and with external fixed resistors are R_(F1)and R_(F2), but in which the connector 23 has been eliminated along withthe excitation voltage source 26. The excitation voltage for the bridge52 has been split and the excitation is provided by two separatedigital-to-analog converters 53 and 54 with digital-to-analog converter53 being connected to the first end of the first fixed resistor R_(F1)and the digital-to-analog converter 54 being connected to the first endof the second fixed resistor R_(F2). These digital-to-analog converters53 and 54 are controlled by a communication link 56 from the computer46.

In place of the independent amplifier 31 there has been provided a highinput impedance differential amplifier 61 which has one input connectedby conductor 62 to the conductor 19. It also has another input connectedby a conductor 63 to a resistor voltage divider network 66 which isconnected to a reference voltage source 68. The reference voltage source68 provides a suitable voltage as for example 2.500 volts to theresistive divider network 66 and to the digital-to-analog converter 53by conductor 69 and to the digital-to-analog converter 54 by theconductor 71. The resistive divider network 66 consists of seriallyconnected resistors R₁ and R₂ which are connected to a ground 72 asshown. It supplies a suitable reference voltage as for example 0.5 voltto the other input of the differential amplifier 61.

In a similar manner, the differential amplifier 76 is substituted forthe independent amplifier 36 and has its input connected by a conductor77 to the conductor 21. The other input of the differential amplifier isconnected by conductor 78 to the divider network 66, and is alsosupplied with a suitable reference voltage as for example 0.5 volts fromthe divider network 66.

Each independent differential amplifier 61 and 76 utilizes twooperational amplifiers to provide a very high input impedance, as forexample 10¹⁴ ohms. Suitable operational amplifiers having low noise andlow offset voltages are available at moderate cost from manymanufacturers. The analog-to-digital converters 41 and 42 can betwo-channel types which in addition to measuring the outputs of each ofthe differential amplifiers 61 and 76, can measure simultaneously theexcitation voltages produced by the digital-to-analog converters 53 and54. These additional channels make it possible to perform self test andcalibration of the circuitry when the circuitry is in use.

The differential amplifiers 61 and 72 have as one of their inputs asdescribed above, a precision-generated offset voltage of 0.5 voltcorresponding to the reference supplied by the reference voltage source68. This offset voltage is subtracted from the transducer voltage, whichis a function of the excitation voltage, which in turn is preciselygenerated by the computer 46 controlled digital-to-analog converters 53and 54 to obtain nominally a transducer voltage of 0.5 volts. Thus, whenthe transducer 12 has been set up properly by the computer 46, andwherein nominal temperature or pressure is being applied to thetransducer, each input to the differential amplifier is 0.5 volt and theoutput of the differential amplifier is 0 volts. Thus, a signal in theoutput of the differential amplifiers 61 and 76 is due to either apressure or temperature change being sensed by the transducer 12. Assoon as the temperature is determined, the computer 46 is capable inreal time of compensating the excitation voltage to eliminate thetemperature component of the transducer signal so that the only outputfrom the differential amplifiers 61 and 76 will be those which representthe pressure being measured by the transducer 12. By compensating forthe offset and temperature voltage component of the transducer signal,it is possible to use very high gain amplification of the pressurecomponent of the signal. This is an important factor because typicallythe pressure component of signal may be a thousand times smaller thanthe combined offset and temperature voltage components. It is for thatreason that all or substantially all of the offset and temperaturevoltage components must be subtracted prior to the large amplificationof the signal.

From the foregoing it can be seen that with a separate excitationvoltage generated by the computer controlled digital-to-analogconverters 53 and 54 driving each leg of the bridge 52 it is possiblefor the computer 46, knowing the nominal resistance for each transducerelement, to set the excitation voltage so that the offset due to thedifference between the transducer resistance from the ideal resistanceR_(F) (3500 ohms) is cancelled out. Thus, at a nominal pressure andtemperature, there will be essentially a zero-volt input to theanalog-to-digital converters 41 and 42. When this is the case, theanalog-to-digital converter input can meet the resolution requirementsover the smaller dynamic range that results from only the temperatureand pressure signals. For the same transducers and operating conditionsused in the above example, the overall input dynamic range is reduced toonly 45 mV. As an added benefit, since the present invention normalizesthe resistance of the worst-case transducer to that of the idealtransducer by adjusting the excitation voltage as a function of theactual resistance, the effective sensitivity of the worst-casetransducer is increased to 3.35 μV/mmHg. The result is that it is onlynecessary to resolve 1 part in 13400 (45 mV/3.35 μV), better than afour-fold improvement, thus making the capabilities of a 15 bitanalog-to-digital converter adequate.

In the circuitry shown in FIG. 2, the computer 46 can also perform realtime tracking of the temperature component of the signal and continuallyadjust the excitation voltages to compensate for temperature changes.The output dynamic range in this case would only be as great as thepressure signal produced by the most sensitive transducer, or about 13mV. This requires accurate measurement to only 1 part in 3900 (13mV/3.35 μV), which can be measured with a 12-bit analog-to-digitalconverted and represents a decrease in dynamic range by a factor morethan 17 with respect to prior art approaches.

Thus, with the circuitry shown in FIG. 2, compensation is provided bythe computer 46. Compensation is provided by varying independently theexcitation voltage to each variable resistor R_(A) and B_(B) of thetransducer 12 as a function of both the initial resistance andtemperature so that the nominal transducer output voltage matches thefixed amplifier reference voltage supplied by the reference source 68.

Alternatively, as shown in FIG. 3, compensation is provided by varyingthe amplifier reference voltages to match the transducer offset voltageswhich are a result of a fixed transducer excitation voltage. Toaccomplish this, the reference voltage from the reference source 68 issupplied to the digital-to-analog converters 53 and 54 as shown whichare under the control of the computer 46 through the communication link56. The outputs 81 and 82 of the digital-to-analog converters 53 and 54are connected to the reference inputs of the differential amplifiers 61and 76 to individually vary the reference voltages thereto. Thereference voltage supplied to each differential amplifier is adjusted tomatch the instantaneous offset and temperature components of the signalfrom the transducer 12 resulting in the outputs from the differentialamplifiers being only the amplified pressure component of the signal.The analog-to-digital resolution requirement would be the same as forthe circuitry shown in FIG. 2. Thus, the computer 46 in FIGS. 2 and 3reads the digitized voltage from the two analog-to-digital converters 41and 42 and computes the pressure and temperature from that information.Also in the circuitry in FIGS. 2 and 3, the computer 46 providesfeedback control to compensate for the deviation from monimal resistancevalues and temperature characteristics of the transducer 12.

From the foregoing it can be seen that there has been provided apiezoresistive transducer circuitry and method accommodating transducervariables in which compensation techniques are utilized to providesignal dynamic range reduction. This has made it possible to reduce thesignal dynamic range into the amplifier and into the analog-to-digitalconverters permitting the computer to compensate for variability in thevariable resistive elements, thereby permitting the computer tocompensate for temperature effects on the variable resistive elements.This invention has also made it possible to provide amplifiers andanalog-to-digital converters which are less expensive because of thedramatically reduced dynamic range requirements. The present inventionmakes it possible to utilize transducer resistors which are veryinsensitive to pressure in comparison to larger predecessors making itpossible to utilize pressure transducers having sensitivities rangingfrom 1.2 to 15 ohms per 1000 ohms per 100 mm of mercury which is anorder of magnitude less than provided by a predecessor of larger scalepressure transducers. The resistive transducer elements utilized in thepresent invention have a construction making it difficult if notimpossible to trim the same to exact end values. Also by way of example,such resistive elements have a nominal resistance of 3500 ohms but havea manufactured tolerance which could vary under no pressure conditionsfrom 2500 to 4500 ohms, or roughly plus or minus 33% variation. Suchresistive elements are also very sensitive to temperature and eachresistive element could increase its resistance by as much as 35% over a100° C. temperature span. The present invention also makes it possibleto overcome another characteristic of such resistive elements in that asthe temperature increases the pressure sensitivity decreases. Decreasein pressure sensitivity was found to be as high as 27% over 100° C.temperature span. In the present invention all these undesirablefeatures of the ultraminiature piezoresistive elements have beenaccommodated.

By overcoming the problems of the ultraminiature piezoresistive pressuretransducers, it is possible to provide pressures, for example makingpressure measurements at the distal extremity of guide wires having anoutside diameter of 0.018" and less and typically 0.014" in diameter.Thus, in making Doppler velocity measurements by an ultrasonictransducer carried by the end of the guide wire it also is possible tomeasure pressure utilizing the present invention and givingcomplimentary velocity and pressure information, which can be of greatvalue in the treatment of coronary artery disease. In the presentinvention it is possible to obtain the pressure variance across lesionsoccurring in a vessel of the patient. For example if there is asignificant pressure gradient on opposite sides of the lesion, thiswould indicate to the physician that a hemodynamically significantlesion was present. In the guide wire, the ultrasonic Doppler transducercan be mounted on the distal extremity so that it would be lookingforward from the tip of the guide wire. The pressure transducer of thepresent invention could be disposed proximal of the ultrasonictransducer making it possible during an angioplasty procedure to measurethe velocity distal to a stenosis then measure the pressure gradientacross the lesion by manipulating the guide wire distally and proximallyto obtain pressure measurements proximal to and distal to the lesionwithout having to cross or recross the lesion with the tip of the guidewire. The tip of the guide wire would always be distal of the lesion.

What is claimed:
 1. Pressure transducer circuitry accommodating pressuretransducer variables for making pressure measurement in a living bodycomprising a microminiature piezoresistive pressure transducer havingfirst and second variable resistive elements, the pressure transducerbeing characterized as having a pressure sensitivity ranging from 1.2 to15 ohms per 1,000 ohms per 100 millimeters of mercury, each having firstand second ends, circuitry coupled to the first and second variableresistive elements of the transducer including third and fourth fixedresistive elements external of the transducer and of known values, eachhaving first and second ends, means for connecting the first ends of thefirst and second resistive elements, means for connecting the secondends of the first and third resistive elements to form a first junction,means for connecting the second ends of the second and fourth resistiveelements to form a second junction, means for supplying excitation tothe first ends of the third and fourth resistive elements, firstamplifier means having an input coupled to the means for connecting thesecond ends of the first and third resistive elements, second amplifiermeans having an input coupled to the means for connecting the secondends of the second and fourth resistive elements, a firstanalog-to-digital converter having an input coupled to the output of thefirst amplifier means, a second analog-to-digital converter having aninput coupled to the output of the second amplifier means, saidanalog-to-digital converters having independent outputs with digitizedvoltages thereon having a pressure component and a temperature componentand computer means coupled to the outputs of the first and secondanalog-to-digital converters for ascertaining the unknown resistances ofthe first and second variable resistive elements as independent variableresistance elements in solving a set of equations having said first andsecond variable resistance elements as unknowns and pressure andtemperature as the remaining unknowns to provide a temperaturecompensated pressure value as measured by the microminiature pressuretransducer.
 2. Circuitry as in claim 1 wherein each of said first andsecond resistive elements has independent undesired characteristicsincluding deviation from nominal resistance values and temperaturecharacteristics resulting in offset voltages at the inputs of said firstand second amplifier means, said circuitry further including feedbackcontrol means controlled by the computer coupled through the third andfourth fixed resistive elements to the first and second variableresistive elements of the transducer for compensating for the offsetvoltage resulting from the resistance and temperature characteristics ofthe first and second variable resistive elements of the transducer. 3.Circuitry as in claim 1, wherein said means for supplying excitation tothe first ends of the third and fourth resistive elements includesexcitation means for supplying a fixed excitation voltage to each of thefirst ends of the third and fourth resistive elements.
 4. Circuitry asin claim 1 wherein said first and second amplifier means are comprisedof first and second independent and separate amplifiers. 5.Piezoresistive pressure transducer circuitry accommodating transducervariables comprising a microminiature pressure transducer having firstand second variable resistive elements, each having first and secondends, circuitry coupled to the first and second variable resistiveelements of the transducer including third and fourth fixed resistiveelements each having first and second ends, means for connecting thefirst ends of the first and second resistive elements, means forconnecting the second ends of the first and third resistive elements,means for connecting the second ends of the second and fourth resistiveelements, means for supplying excitation to the first ends of the thirdand fourth resistive elements, first amplifier means having an inputcoupled to the means for connecting the second ends of the first andthird resistive elements, second amplifier means having an input coupledto the means for connecting the second ends of the second and fourthresistive elements, a first analog-to-digital converter having an inputcoupled to the output of the first amplifier means, a secondanalog-to-digital converter having an input coupled to the output of thesecond amplifier means, said analog-to-digital converters having outputswith digitized voltages thereon, a computer, means coupling the computerto the outputs of the first and second analog-to-digital converters sothat the computer reads the digitized voltages and computers thepressure therefrom each of said first and second resistive elementshaving independent undesired characteristics including deviation fromnominal resistance values and temperature characteristics resulting inoffset voltages at the inputs of said first and second amplifier meansand further including feedback control means controlled by the computercoupled to the first and second variable resistive elements of thetransducer for compensating for the offset voltages resulting from theresistance and temperature characteristics of the first and secondvariable resistive elements of the transducer, said first and secondamplifier means including first and second differential amplifiers, eachhaving a reference input and means for supplying a fixed referencevoltage to the reference input of the first and second differentialamplifiers, said means for supplying excitation to the first ends of thethird and fourth resistive elements including means coupled to thecomputer for supplying a varying independent excitation voltage to eachof the first ends of the third and fourth resistive elements so that theoutput voltage components of the transducer dependent on temperature anddeviation from nominal resistance values will substantially match thefixed reference voltages supplied to the first and second differentialamplifiers.
 6. Circuitry as in claim 5 wherein said means coupled to thecomputer for supplying a varying independent excitation voltage includesa first digital-to-analog converter connected to the first end of thethird resistive element and a second digital-to-analog converterconnected to the first end of the fourth resistive element.
 7. Circuitryas in claim 5 wherein said means for supplying a fixed reference voltageto the reference inputs of the first and second differential amplifiersincludes a voltage divider network.
 8. Piezoresistive pressuretransducer circuitry accommodating transducer variables comprising amicrominiature pressure transducer having first and second variableresistive elements, each having first and second ends, circuitry coupledto the first and second variable resistive elements of the transducerincluding third and fourth fixed resistive elements each having firstand second ends, means for connecting the first ends of the first andsecond resistive elements, means for connecting the second ends of thefirst and third resistive elements, means for connecting the second endsof the second and fourth resistive elements, means for supplyingexcitation to the first ends of the third and fourth resistive elements,first amplifier means having an input coupled to the means forconnecting the second ends of the first and third resistive elements,second amplifier means having an input coupled to the means forconnecting the second ends of the second and fourth resistive elements,a first analog-to-digital converter having an input coupled to theoutput of the first amplifiers means, a second analog-to-digitalconverter having an input coupled to the output of the second amplifiermeans, said analog-to-digital converters having outputs with digitizedvoltages thereon, a computer and means coupling the computer to theoutputs of the first and second analog-to-digital converters so that thecomputer reads the digitized voltages and computes the pressuretherefrom, said means for supplying excitation to the first ends of thethird and fourth resistive elements including excitation means forsupplying a fixed excitation voltage to each of the first ends of thethird and fourth resistive elements, said first and second amplifiermeans including first and second differential amplifiers, each having areference input means coupled to the computer for supplying a varyingindependent reference voltage to each reference input to the first andsecond differential amplifiers to substantially match the offset voltageand temperature characteristics of the first and second variableresistive elements.
 9. Circuitry as in claim 8 wherein said meanscoupled to the computer for supplying a varying independent referencevoltage to each reference input to the first and second differentialamplifiers includes first and second digital-to-analog converters.
 10. Amethod for utilizing circuitry with a microminiature piezoresistivepressure transducer for making pressure measurements in a living body inwhich the transducer is characterized as having a pressure sensitivityranging from 1.2 to 15 ohms per 1,000 ohms per 100 millimeters ofmercury and is provided with first and second variable resistiveelements, each having first and second ends, circuitry coupled to thefirst and second resistive elements including third and fourth fixedresistive elements each having first and second ends, means forconnecting the second ends of the first and third resistive elements toform a first junction, means for connecting the second ends of thesecond and fourth resistive elements to form a second junction, andmeans including a computer supplying excitation voltages to the firstends of the third and fourth resistive elements so that first and secondanalog signals appear respectively on the first and second junctions,the method comprising amplifying the first and second analog signals toprovide first and second amplified analog signals, converting the firstand second amplified analog signals to first and second digital signalsand supplying the first and second digital signals to the computer andthen utilizing the computer to ascertain the unknown resistances of thefirst and second variable resistive elements as independent variableresistive elements in solving a set of equations having said first andsecond variable resistive elements as unknowns and pressure andtemperature as the remaining unknowns to provide a temperaturecompensated pressure value being measured by the microminiature pressuretransducer.
 11. A method as in claim 10 for reducing the requirements ofthe dynamic range in converting the first and second amplified analogsignals to first and second digital signals where the first and secondvariable resistive elements have independent undesired characteristicsincluding deviation from nominal resistance values and temperaturecharacteristics, the method further including the steps of using firstand second differential amplifiers having references for amplifying thefirst and second analog signals, supplying an offset voltage to each ofthe references of the differential amplifiers, ascertaining thetemperature of the transducer and adjusting the outputs of the first andsecond differential amplifiers so that the outputs only reflect thepressure being measured by the transducer.
 12. A method as in claim 11wherein adjusting the outputs of the first and second differentialamplifiers includes the step of varying independently the excitationvoltages to the first ends of the third and fourth fixed resistiveelements to compensate for the offset voltages and for the temperaturecharacteristics of the first and second variable resistive elements. 13.A method as in claim 11 further including the steps of utilizingdigital-to-analog converters for connecting the computer and thereference voltage to the first ends of the third and fourth fixedresistive elements.
 14. A method as in claim 11 further including thesteps of utilizing digital-to-analog converters for connecting thecomputer and the reference voltage to the differential amplifiers forproviding the references for the differential amplifiers.
 15. A methodfor utilizing circuitry with an microminiature piezoresistive pressuretransducer for making pressure measurements in a living body in whichthe transducer is provided with first and second variable resistiveelements, each having first and second ends, said first and secondvariable resistive elements having independent undesired characteristicsincluding deviation from nominal resistance values and temperaturecharacteristics, circuitry coupled to the first and second resistiveelements including third and fourth fixed resistive elements each havingfirst and second ends, means for connecting the second ends of the firstand third resistive elements to form a first junction, means forconnecting the second ends of the second and fourth resistive elementsto form a second junction, means including a computer supplyingexcitation voltages to the first ends of the third and fourth resistiveelements so that first and second analog signals appear respectively onthe first and second junctions, the method comprising amplifying thefirst and second analog signals by the use of first and seconddifferential amplifiers having references, converting the first andsecond amplified analog signals to the first and second digital signals,supplying the first and second digital signals to the computer,utilizing the computer to calculate the pressure being measured by thetransducer, causing the computer to control the circuitry to compensatefor the offset voltages resulting from the resistance and temperaturecharacteristics of the first and second variable resistive elements ofthe transducer by varying independently the references utilized duringamplification of the first and second analog signals.